If the income elasticity of demand for store brand macaroni and cheese is - 3.00, this means that

Income elasticity of demand or price elasticity of demand shows how responsive the demand of an item is to price changes. Now - 3.00 is elastic, because an elasticity that is greater than 1 means that it is elastic. But what does the negative mean?

The negative sign indicates that price and demand are inversely proportional. That means that as the price goes up, the demand goes down. Vice versa, as the price goes down the demand goes up.